1. Field of the Invention
This invention relates to a layer structure oxide as typified by oriented LiCoO.sub.2 which is suitable for use in a secondary cell as a positive electrode.
2. Prior Art
From the past, layer structure or lattice compounds have drawn attention for their unique properties. These compounds allow intercalation of a guest substance between their layers because of their layer structure and exhibit electrical anisotropy because of their crystallographic anisotropy. Exemplary applications utilizing such nature include lithium cell positive electrodes utilizing intercalation of lithium ions and conductor wires using oriented superconducting oxides.
Such layer structure compounds have a structure as shown in FIG. 8 which depicts the structure of LiNiO.sub.2 as a typical example. Ions migrate between the layers shown in FIG. 8 in a two-dimensional manner, allowing for intercalation of Li ions. In an application as the secondary cell positive electrode material, it is desired to make use of this feature as much as possible. Ideally, the compound should exist continuous, for example, as a single crystal as long as necessary. If the compound takes a polycrystalline form, the boundary region becomes a barrier against ion transport.
It was proposed to use a layer structure composite oxide such as AMO.sub.2 wherein A is Li or Na and M is a transition metal such as Co and Mn as the positive electrode material of a lithium secondary cell. For example, Japanese Patent Publication (JP-B) No. 59507/1988 discloses Li.sub.x M.sub.y O.sub.2 wherein M is Co or Ni, x is 0.8 or less, and y is approximately equal to 1 and wherein Li.sup.+ cation vacancies are created in the ion conductor by extraction of Li.sup.+ cations. Japanese Patent Application Kokai (JP-A) NO. 253162/1992 discloses LiCoO.sub.2 wherein part of Co is substituted by at least one element selected from the group consisting of Pb, Bi, and B, with the composition LiCo.sub.0.90 Pb.sub.0.10 O.sub.2 and the corresponding compositions wherein Pb is replaced by Bi or B being exemplified therein.
Also JP-A 290849/1993 discloses the preparation of LiCoO.sub.2 by mixing a source powder with a binder, granulating and firing. It is attempted to employ the granulating step prior to the firing step in order to increase the sintered grain size (which grains are not always single crystals), thereby reducing a self discharge rate and achieving some improvements. Although the reason why properties are improved is not described in JP-A 290849/1993, it is presumed that the self discharge rate lowers with an increasing grain size because the proportion of lithium ions that diffuse outward from the sintered grains is reduced. If this concept is applied to layer structure compounds, it is expected that their properties can be improved by increasing the size of crystallites.
We thus found that in order to make use of the characteristic feature of a layer structure compound as a positive electrode material, it is important that the compound have an increased crystallite size and be continuous as long as necessary. It is important to promote conversion of individual grains into crystallites and at the same time, to increase the size of crystallites. Based on this finding, we made efforts to develop a layer structure compound which takes a crystallite form not found in the prior art.